1. Field of the Invention
The present invention relates to a cellulose acylate film, and a polarizing plate and liquid crystal display device using the film.
2. Description of the Related Art
Cellulose acylate films have appropriate water permeability and high optical isotropy (low retardation values), and thereby have been widely used as protective films for polarizing plates of liquid crystal display devices.
Recently proposed are methods for preparing cellulose acylate films having optical compensatory function in addition to function as polarizing plate protective films by forming phase difference in the films.
In the case of using the cellulose acylate films as optical compensatory films for VA mode liquid crystal display devices, expanding its market rapidly, not only the in-plane retardation (hereinafter referred to as Re) but also the retardation in the thickness direction have to be high.
As methods for increasing the retardation in the thickness direction (hereinafter referred to as Rth) of the cellulose acylate films, methods of reducing the total acylation degree of cellulose acylate, methods of adding a retarder, etc. have been known. With respect to the latter methods, a cellulose acylate film containing a triazine compound added is disclosed in JP-A-2003-344655.
However, the cellulose acylate films have a certain level of water absorptivity, and thus the retardation of the films is changed by environmental humidity. This causes a problem that, when the cellulose acylate films are used as the optical compensatory films of the liquid crystal display devices, the image qualities of the devices are changed depending on the environmental humidity. Therefore, there has been a strong demand for improving the change.